In conventional Kraft mill pulping of cellulosic fibrous material, generally wood, sodium sulphide and sodium hydroxide are used as the active pulping chemicals. In addition, the white liquor usually contains sulphur-oxygen-salts, such as, sodium sulphate, sodium thiosulphate and sodium sulphite. The sulphur-oxygen salts take no part in the pulping process and represent a dead load which cycles within the system. Only about 60 to 70% of the total sulphur content of the white liquor is present as the pulpingly-active sodium sulphide.
It is known that polysulphide cooking of wood has advantages over the conventional Kraft mill cooking, including an increased yield in pulp recoverable from the wood, usually up to 10% on pulp, improved pulp freeness with a consequently decreased energy requirement for pulp beating, and decreased corrosion.
Various suggestions have been made for the production of polysulphide pulping liquor and one of the recent suggestions is to oxidize the sodium sulphide contained in white liquor and utilization of the resulting liquor as the pulping liquor. While this procedure is successful in converting some sodium sulphide to sodium polysulphide, sodium sulphide also is oxidized to sodium thiosulphate and the fraction of the total sulphur of the pulping liquor which is present as sodium polysulphide is quite limited, rarely exceeding about 20% of the total sulphur. This pulping liquor, therefore, contains a high proportion of the total sulphur as pulpingly-inert sulphur-oxygen salts.
The limited polysulphide content of the pulping liquor is a considerable drawback since the liquor has a high sulphidity, which leads to sulphur emissions and a high chemical make-up requirement.